Recent years have seen a popular utilization of thinned and power-saving liquid crystal display (LCD) panels as monitors for televisions and personal computers in lieu of prior art cathode ray tubes. Display of LCD panels is indicated by an illuminating device such as a backlight disposed behind LCD panels because they cannot emit a light themselves. A typical backlight for LCD panel usually includes a cold cathode fluorescent lamp (CCFL) of electric property to need application of high AC voltage thereto in the order of one thousand and several hundreds volt at the beginning of lighting and several hundreds volt after lighting. Most recently, expansion in size of LCD panels tends to promote smaller and longer CCFL tubes, requiring further increase in applied voltage and consumption power.
Here, when a discharge lump lighting device applies high AC voltage to CCFL with high frequency, undesirable arc discharge may possibly occur in a small space that may be formed due to looseness in connector or the like, disconnection of wiring pattern or crack in solder. For example, CCFL is more frequently connected to a secondary winding in a transformer for electric driving of CCFL, and the winding is formed of a narrow wire to well increase the number of turns. In this arrangement, arc discharge may be generated by disconnection in the secondary winding when it is subject to mechanical tension or nipping solder for soldering to terminals. In another aspect, without soldering terminals in transformer on a printed circuit board in a normal condition, it fails to form a firm electrical contact between terminals of transformer and wiring pattern on printed circuit board, and this may result in arc discharge at wrong contact portions. Otherwise, if any damage is incurred to wiring pattern or if any mechanical load is applied on wiring pattern due to thermal deformation of circuit board, wiring pattern is cut off while arc discharge may possibly develop at the disconnection portion. In addition, when either terminal or both terminals of CCFL are not appropriately inserted into a connector with contact failure, arc discharge may also happen at an area of imperfect electrical contact.
FIG. 10 illustrates a lighting system 100 by way of example of a prior art discharge lamp lighting device which comprises a power supply circuit (a power supply) 120 for boosting input voltage Vin applied on an input terminal Tin to apply a raised voltage on one end of a cold cathode fluorescent lighting tube (CCFL) 110, a protective circuit 140 inclusive of a current control circuit or current controller 210, an extinction detection circuit or extinction detector 220 and a forced outage circuit 230, and a current detection circuit or current detector 130 for converting electric current through CCFL 110 into a corresponding voltage to protective circuit 140. Current controller 210 serves to control the voltage impressed from power supply 120 to CCFL 110 to render an effective value in electric current flowing into CCFL 110 constant or consistent in response to detected voltage from current detector 130. Extinction detector 220 serves to detect extinction or disappearance of electric current through CCFL 110 in response to detected voltage from current detector 130 to generate an extinction detection signal to forced outage circuit 230. In other words, extinction detector 220 can appreciate lights out of CCFL 110 by sensing extinction of electric current through CCFL 110. When extinction detector 220 produces an extinction detection signal, forced outage circuit 230 functions to forcibly and temporarily stop operation of power supply 120. Lighting system 100 shown in FIG. 10 has a notable advantage that forced outage circuit 230 can forcibly and temporarily suspend operation of power supply 120 by forwarding an extinction detection signal from extinction detector 220 to forced outage circuit 230 in protective circuit 140 upon disappearance of electric current through CCFL 110 due to lighting failure of CCFL 110, disconnection of CCFL 110 from related connector or the like to thereby avert occurrence of arc discharge at a location of bad connection in CCFL 110.
Also, FIG. 11 demonstrates another lighting system 200 as a further prior art discharge lamp lighting device which comprises a CCFL 110 and a drive device 111 for activating CCFL 110. Drive device 111 comprises a power supply circuit or power supply 141, a current detection circuit or current detector 142, a peak hold circuit 143 and a protective circuit 144. Protective circuit 144 comprises a current control circuit 161, an extinction detection circuit or extinction detector 162, overcurrent detection circuit or overcurrent detector 163 and a forced outage circuit 164.
FIG. 12 indicates voltages appearing at several locations in lighting system 200 during its operation. For example, when arc discharges repeatedly emerge n− times during a period from point t1 to tn in time as shown in FIG. 12(A) because of contact failure between CCFL 110 and related connector not shown in the drawings, spike-like surge voltages are superimposed on detected voltage from current detector 142 each time arc discharge occurs. These surge voltages of n-times cause to gradually electrically charge a voltage-hold capacitor (not shown) in peak hold circuit 143 to moderately increase charged voltage in voltage-hold capacitor as is understood by FIG. 12(B). When charged voltage in voltage-hold capacitor comes to a voltage level Vref2 regulated by a reference power source (not shown) in overcurrent detector 163 at point tn in FIG. 12(B), overcurrent detector 163 produces an output signal of high voltage level as shown in FIG. 12(C) to forced outage circuit 164 which then forwards a stop signal of high voltage level to power supply 141. Accordingly, power supply 141 ceases its operation as shown in FIG. 4(D) to halt supply of high AC voltage from power supply 141 to CCFL 110. For that reason, current flow into CCFL 110 is ceased, and therefore, current detector 142 finds zero potential in detected voltage as shown in FIG. 12(A). At this time, voltage-hold capacitor in peak hold circuit 143 maintains charged voltage value Vref2 at point tn until a reset signal of high voltage level shown in FIG. 12(E) is supplied to a reset terminal Tr of peak hold circuit 143 to retain the outage condition for disconnecting supply of voltage from power supply 141 to CCFL 110 although a temporal contact is completed between CCFL 110 and connector. Then, when a reset signal of high voltage level as in FIG. 12(E) is supplied to reset terminal Tr of peak hold circuit 143 at point t11, voltage-hold capacitor in peak hold circuit 143 is electrically discharged to an approximately zero voltage as shown in FIG. 12(B) to switch output of overcurrent detector 163 from high to low voltage level as seen in FIG. 12(C). This allows power supply 141 to again feed high AC voltage to CCFL 110 so that current detector 142 again produces a detection voltage as in sinusoidal wave shown in FIG. 12(A).
When several arc discharges occur due to contact failure of CCFL 110 in lighting system 200 shown in FIG. 11, they cause concomitant surge voltages each time arc discharge is generated while each surge voltage is superimposed on detected voltage from current detector 142 and also impressed on voltage-hold capacitor within peak-hold circuit 143. In this condition, when voltage-hold capacitor is electrically charged to a predetermined potential level, overcurrent detector 163 in protective circuit 144 issues a stop signal of high voltage level to power supply 141 through forced outage circuit 164 to cease feed of high AC voltage from power supply 141 to CCFL 110 for protection of CCFL 110 from overheating by arc discharges attributable to a contact-failure location. For example, the following Patent Document 1 discloses a discharge lamp lighting device having the substantially same configuration as lighting systems 100 and 200 shown in FIGS. 10 and 11.
[Patent Document 1] Japanese Patent Disclosure No. 2005-340023